3.1.3.2.1.

Controlled/Uncontrolled Variation

Two trend plots

The two figures below are two trend plots from two different oxide
growth processes. Thirty wafers were sampled from each process: one
per day over 30 days. Thickness at the center was measured on each wafer.
The x-axis of each graph is the wafer number and the y-axis
is the film thickness in angstroms.

Examples of"in control" and "out of control" processes

The first process is an example of a process that is "in control"
with random fluctuation about a process location of approximately 990.
The second process is an example of a process that is "out of control"
with a process location trending upward after observation 20.

This process exhibits controlled variation.
Note the random fluctuation about a constant mean.

This process exhibits uncontrolled variation.
Note the structure in the variation in the form of a linear trend.